1. Field of the Invention
This invention relates to an air-fuel ratio control system for internal combustion engines, and more particularly to an air-fuel ratio control system for an international combustion engine which has an exhaust gas-purifying device with an absorbent for absorbing nitrogen oxides, arranged in the exhaust system.
2. Prior Art
When an internal combustion engine operates in a condition where the air-fuel ratio of a mixture supplied to the engine is set to a leaner value than a stoichiometric air-fuel ratio to carry out so-called lean-burn control, it is likely that an increased amount of nitrogen oxides (hereinafter referred to as "NOx") is emitted from the engine. To overcome this disadvantage, it is conventionally employed to provide an exhaust gas-purifying device accommodating a NOx absorbent for absorbing NOx arranged in the exhaust system of the engine, to thereby carry out purification of exhaust gases emitted from the engine. The NOx absorbent has such a characteristic that it absorbs NOx when the air-fuel ratio is leaner than the stoichiometric air-fuel ratio and accordingly the concentration of oxygen present in exhaust gases is relatively high, i.e. the amount of NOx is large (hereinafter referred to as "the exhaust gas lean state"), whereas it desorbs the absorbed NOx when the air-fuel ratio is richer than the stoichiometric air-fuel ratio and accordingly the concentration of oxygen present in exhaust gases is low, i.e. the amount of HC and CO is large (hereinafter referred to as "the exhaust gas rich state"). Therefore, in the exhaust gas rich state, the exhaust gas-purifying device with the NOx absorbent functions to reduce NOx desorbed from the NOx absorbent to a nitrogen gas by reaction with HC and CO, which is emitted into the air, and oxidize HC and CO into steam and carbon dioxide, which are also emitted into the air.
The NOx absorbent, however, has a limited capacity for absorbing NOx, and therefore the lean-burn control cannot be continued over a long time period. To cope with this inconvenience, there is conventionally known an air-fuel ratio control method, for example, from Japanese Laid-Open Patent Publication (Kokai) No. 6-294319, which temporarily enriches the air-fuel ratio in order to desorb NOx which has been absorbed by the NOx absorbent, for reduction of the thus desorbed NOx. In the present specification, this temporary enrichment of the air-fuel ratio for desorbing NOx will be referred to as "reduction enrichment".
According to this air-fuel ratio control method, the degree of reduction enrichment is set to a larger value as the amount of exhaust gases emitted per unit time is smaller. In other words, the degree of reduction enrichment is set to a smaller value as the amount of exhaust gases is larger. This setting is based on the ground that when the amount of exhaust gases emitted per unit time is smaller and accordingly the amount of HC and CO present in the exhaust gases is smaller, NOx desorbed from the NOx absorbent cannot be reduced to a sufficient degree if the degree of reduction enrichment or enrichment of a mixture supplied to the engine is constant.
Further, there is conventionally known a control method, for example, from Japanese Laid-Open Patent Publication (Kokai) No. 7-54695, which, in changing the air-fuel ratio of a mixture supplied to an internal combustion engine with a three-way catalyst arranged in the exhaust system, from a stoichiometric air-fuel ratio to a leaner value, temporarily sets the air-fuel ratio to a richer value than the stoichiometric air-fuel ratio and then changes the air-fuel ratio to a leaner value, to thereby reduce the emission amount of NOx immediately after the change of the air-fuel ratio to the leaner value. According to this method, by temporarily enriching the air-fuel ratio, oxygen stored in the three-way catalyst is released to enhance the oxygen storage capacity of the three-way catalyst, to thereby reduce the emission amount of NOx immediately after the change of the air-fuel ratio to a leaner value.
The method according to Japanese Laid-Open Patent Publication (Kokai) No. 6-294319, however, has the following inconvenience: That is, when a large amount of exhaust gases per unit time is emitted from the engine, the temperature of the NOx absorbent rises, which leads to an increase in the amount of NOx desorbed from the NOx absorbent. If the degree of enrichment is then set to a smaller value, NOx cannot be reduced to a sufficient degree. Further, a larger amount of exhaust gases emitted per unit time, i.e. a larger exhaust gas flow rate (volume/time) means a higher exhaust gas flow velocity (volume/(time.times.cross sectional area)), which results in insufficient contact between HC and CO and the NOx absorbent (catalyst). This also leads to insufficient reduction of NOx. As a result, the emission amount of NOx unfavorably increases as the exhaust gas flow rate increases.
Further, if the method according to Japanese Laid-Open Patent Publication (Kokai) No. 7-54695 is applied to the air-fuel ratio control of an internal combustion engine provided with the exhaust gas-purifying device accommodating the NOx absorbent, to change the air-fuel ratio from a stoichiometric value to a leaner value, a similar inconvenience to that mentioned above can occur. That is, when the reduction enrichment is first carried out and then the air-fuel ratio is controlled to a leaner value, if the degree of reduction enrichment is made constant irrespective of the exhaust gas flow rate or the degree of reduction enrichment is made smaller as the exhaust gas flow rate increases, the emission amount of NOx unfavorably increases.